The growth of low defect single crystals has been the subject of considerable research in for example the semiconductor industry. Such crystals are an essential precursor in the fabrication of a vast variety of semiconductor devices.
The Czochralski seed-pulling technique for growing single crystals is well known (eg Z Physik. Chem. (Liebzig) 92, 219 (1918)). By this technique a seed crystal is brought into contact with molten material (melt) to facilitate further crystallisation. The crystal so produced is drawn from the melt as it grows.
However, for an incongruently solidifying melt of more than one component (where the liquidus and solidus temperatures are different) this method does not give rise to uniform composition throughout the crystal. This is because the composition of crystal which crystalises from a given melt is different in composition from that melt (see, for example, S Glasstone, "The Elements of Physical Chemistry" p384, published by Macmillan, 1956). Hence, for a closed system, the composition of the melt, and hence the growing crystal changes as crystallisation proceeds.
In order to produce crystals of uniform composition, the double crucible technique was developed (eg Journal of Applied Physics, 29, no. 8, (1958) pp1241-1244 and U.S. Pat. No. 5,047,112).
Typically, the apparatus of this technique comprises an outer crucible containing melt of the same composition as the crystal to be grown. An inner crucible floats on the melt in the outer crucible and a small channel through the bottom or side wall of the inner crucible allows melt to flow in from the outer crucible.
The crystal is drawn from the melt in the inner crucible (which is replenished from the outer crucible via the channel) and, under equilibrium conditions, has the same composition as the molten material in the outer crucible.
Typically, the respective melting points of the contents of each crucible are different and an essential feature of the double crucible technique (as applied to an incongruently solidifying melt) is that a temperature differential must be maintained between the outer and inner crucible: the temperature at which the crystal is drawn from the melt in the inner crucible is lower than that required to keep the melt in the outer crucible molten.
For a given melt composition, the temperature differential achieved depends, inter alia, on the dimensions of each crucible (including wall and base thickness), the thermal conductivity of the crucible material, and the power and configuration of the heat source used to achieve melting.
One consequence of the difference in melting points between the respective contents of the two crucibles and of the temperature differential maintained between the two crucibles, is that the melt tends to freeze as it passes through the channel to the inner crucible. The resulting blockage is one of the major problems encountered when using the floating crucible technique with incongruently solidifying melts.
The tendency for freezing to block the hole can be reduced by increasing its diameter but this increases the tendency for diffusion of the melt in the inner crucible back into the outer crucible (see for example G R Blackwell, Solid State Electronics 7 105 (1964)). When this occurs, the composition of melt in the outer crucible no longer remains constant and hence the composition of the crystal produced is no longer uniform.
Another feature of the floating crucible technique is that the inner crucible must be maintained concentric with the outer crucible if a symmetrical temperature distribution across the system is to be maintained. If the inner crucible drifts toward the walls of the outer crucible then undesirable localised freezing of the outer melt is likely.
In response to this problem, inner crucibles have been designed which incorporate spacers or fenders to ensure they remain substantially concentric with the outer crucible. However, this design gives rise to another problem, namely that of sticking of the inner crucible. As crystal growth proceeds, the level of melt in the outer crucible falls and the floating inner crucible falls with it. With the incorporation of fenders or spacers on the inner crucible there is an increased tendency for the inner crucible to stick giving rise to uneven crystal growth or, in extreme cases, halting the growth process altogether.
Another disadvantage of the double crucible technique is that, after crystal growth is complete, residual melt may cool and solidify in the double crucible assembly fusing the two crucibles together. This causes many crucibles to be broken or discarded. Ideally, the two crucibles would be separated before cooling and solidification takes place but crystal growth normally takes place at elevated temperatures and the double crucible assembly is typically maintained in an enclosed, inert atmosphere during growth in order to prevent the components of the melt being oxidised or otherwise degraded. Therefore manual separation of the two crucibles before cooling is difficult without exposing the residual melt to the outside atmosphere.
Another problem commonly encountered when using the double crucible technique is that after charging both crucibles with appropriate melt, gas is often trapped in the channel for allowing melt to pass into the inner crucible and surface tension at the melt-gas interface prevents flow of melt from the outer to the inner crucible.